Ar ar dating of basalts
Dating of movement on fault systems is also possible with the Ar method.
Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C.
Stepwise thermal and AF demagnetization generally isolated a high temperature component (HTC) of magnetization for both Shovon and Arts-Bogds basalts, eventually following a low temperature component (LTC) in some samples.
The HTC directions display normal polarity, consistent with the Cretaceous Long Normal Superchron.
The abundance of Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature.
However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral.
The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace.
Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released.
The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time.
Ages between 436 and 42 ka were found on lavas between the depths of 1650 and 468 m.
All subaerial ages are consistent with their stratigraphic position and allow us to derive a lava accumulation rate of 3.07±0.44 mm/a, which is within the range of rates determined in high OIBs.